The present invention relates generally to a semiconductor memory device, and more particularly to a negative voltage generating device.
A semiconductor dynamic random access memory (DRAM) device includes a plurality of cell capacitors for storing data and a plurality of cell transistors for controlling the input of data to be stored and the output of stored data. Operations which the DRAM performs include data storage, data read, and data refresh for the cell capacitor. In a data refresh, the refresh period is affected to a large degree by leakage current, and accordingly the reduction of leakage current has become an area of much importance.
Leakage current can typically be classified into junction leakage current and channel leakage current.
Junction leakage current is generated by defects in the junction boundary of a cell transistor, and the channel leakage current is a leakage current flowing through the channel of a cell transistor.
The junction leakage current can be reduced by reducing the ion concentration of the channel; however, reducing the ion concentration of the channel causes an increase in the channel leakage current. The channel leakage current can be reduced by increasing the threshold voltage of the cell transistor; however, increasing the threshold voltage of the cell transistor causes the junction leakage current to increase.
In order to reduce both of these leakage currents, negative wordline driving can be used. In negative wordline driving, a high voltage VPP is provided when enabling a wordline, and a negative voltage VNN lower than the ground level is provided when disabling a wordline. The voltage used as the negative voltage lower than the ground level is the voltage used to reverse bias a substrate (that is, the voltage commonly referred to as a backbias voltage VBB in order to distinguish it from the negative voltage VNN).
Negative wordline driving can improve refresh characteristics along with an improvement in the characteristics of other AC parameters. In particular, the refresh time can be reduced, the VPP burden can be reduced when a low level operation voltage Vcc is used, and the Write Recovery Time (tWR) can be improved, and therefore negative wordline driving has been widely used.
FIG. 1 is a block diagram of a voltage generating device for generating a negative voltage VNN.
Referring to FIG. 1, a voltage generating device includes a VNN level detector 10, an oscillator 20, and a pumping unit 30.
The VNN level detector 10 receives a negative voltage VNN, detects the level of the negative voltage, and outputs an oscillator enable signal OSCEN. The oscillator 20 receives the oscillator enable signal OSCEN and generates a pulse signal OSC when the enable signal OSCEN is enabled. The pumping unit 30 receives the pulse signal OSC and performs charge pumping.
The negative voltage generating device shown in FIG. 1 generates a regulated negative voltage using a negative feedback operation. When the negative voltage VNN increases, the level detector 10 enables the enable signal OSCEN, and the oscillator 20 is enabled by the enable signal OSCEN. The negative voltage VNN level is then gradually reduced by the charge pumping until the oscillator 20 is disabled.
The VNN level detector 10 can be configured as shown in FIG. 2. The detector of FIG. 2 includes PMOS transistors M1 and M2, and inverters INV1 and INV2. Referring to FIG. 2, when the negative voltage VNN increases, the source-drain equivalent resistance of the PMOS transistor M2 increases; and thereby, the voltage of node A increases. When the voltage of node A reaches the “trip point” of the inverter, the output signal OSCEN is raised to a high level to enable the oscillator 20, and the enabled oscillator 20 drives the pumping unit 30.
Referring back to FIG. 1, the pumping unit 30 includes a capacitor C and two diodes D1 and D2. When the pulse signal OSC output from the oscillator 20 is high, the voltage of node B is clamped into a threshold voltage Vth higher than a ground voltage by the diode D1, and the capacitor C is charged with a positive voltage VDD. When the pulse signal OSC is low, the capacitor C supplies negative charges through the diode D2.
FIG. 3 shows a waveform of the negative voltage VNN generated using the configuration of FIGS. 1-2. Referring to FIG. 3, it can be appreciated that when using the configuration of FIGS. 1 and 2, the negative voltage VNN has a relatively large ripple.
The negative voltage generated as above is biased into a negative voltage of a wordline. The backbias voltage VBB provided to the wordline is generated using the same method as that used to generate the negative voltage to be biased into the word line.
FIG. 4 shows a typical sub wordline driver that receives the negative voltage VNN.
Referring to FIG. 4, the CMOS sub wordline driver includes one PMOS transistor M3 and two NMOS transistors M4, M5.
One terminal of the PMOS transistor M3 is connected to a sub wordline driving voltage FX, and the other terminal is connected to a sub wordline SWL. The gate of the PMOS transistor M3 is connected to an inverted main wordline MWLB. One terminal of the NMOS transistor M4 is connected to the sub wordline SWL and the terminal of the PMOS transistor M3 connected to the sub wordline SWL. The remaining terminal of the NMOS transistor M4 is connected to the negative voltage VNN. The gate of the NMOS transistor M4 is connected to the inverted main wordline MWLB (as in the PMOS transistor M3). One terminal of the NMOS transistor M5 is connected to the sub wordline SWL, and the other terminal thereof is connected to the negative voltage VNN.
The backbias voltage VBB is applied as the substrate voltage to the NMOS transistors M4, M5.
Parasitic diodes D3, D4 are formed between the substrate (P-type impurity) and the source (N-type impurity) of the NMOS transistors M4, M5. When the ripple of the backbias voltage VBB applied to the substrate and the negative voltage VNN applied to the source is large, the voltage difference of the backbias voltage VBB and the negative voltage VNN may be higher than the threshold voltage of the parasitic diodes at certain time points. If the parasitic diodes D3, D4 are turned on, undesired leakage current flows thereby causing malfunctions in the semiconductor device.
The backbias voltage VBB is generated using aforementioned method. The backbias voltage VBB and the negative voltage VNN applied to the sub wordline are operated independently, and thus the large ripple of the backbias voltage VBB may be higher than the negative voltage VNN at a certain time point. FIG. 5 confirms that the potential difference of the backbias voltage VBB and the negative voltage VNN can be higher than Von
In order to prevent malfunction in the semiconductor device, a stable backbias voltage having a small ripple is needed.